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Spatial stock assessment models are recognised as increasingly important for estimation of stock status and a sustainable exploitation rate. The inclusion of movement between spatial units within a model is difficult because the data requirements are high. However, for populations with low levels of spatial exchange it is possible to reduce the data requirements by distributing information on biological parameters between neighbouring units, or units with shared environmental conditions. This can allow spatial modelling to be applied even in data-limited situations. We develop this approach here through application to orange roughy (Hoplostethus atlanticus) subpopulations inhabiting neighbouring seamounts in the South Pacific. Despite limited data for each seamount, we were able to simultaneously fit multiple, localised, process-based models of the depletion dynamics. This was achieved by sharing information on the unexploited population size via known environmental covariates, with the relationship estimated in a hierarchical and integrated manner during the model fit. Cross-validation demonstrated that this approach can compensate for a lack of seamount-specific abundance data and improve ability of the model to estimate localised depletions.

Life-span estimates for orange roughy ( Hoplostethus atlanticus ) range from ~20 years to well over 100 years. In this study, an improved lead–radium dating technique provided independent age estimates from sagittal otoliths. This technique used the known properties of radioactivity for lead-210 and radium-226 to determine the validity of fish age estimates. An improvement to lead–radium dating using mass spectrometry allowed the use of smaller samples than previously possible; therefore, an application was made to otolith cores, the first few years of otolith growth. This approach circumvented the use of whole otoliths and alleviated many of the assumptions that were necessary in previous lead–radium dating applications. Hence, it was possible to critically evaluate lead–radium dating as a tool in fish age validation. The measurement of lead–radium ratios for a series of age groups that consisted of otolith cores, grouped based on growth-zone counts from thin sections, showed a high degree of correlation to the expected lead–radium ingrowth curve. This finding provided support for age estimation procedures using thin otolith sectioning. As independent estimates of age, the results indicated that fish in the oldest age group were at least 93 years old, providing robust support for a centenarian life span.

The history of orange roughy (Hoplostethus atlanticus) stocks, primarily in New Zealand and Australia, is commonly used as an example of the inability to manage fisheries resources. We review the history and status of the New Zealand orange roughy fishery and show that the total loss of potential biological yield from overfishing is no more than 8.3% (1260 tonnes (t)·year–1) of the potential yield. The losses from underfishing are estimated to be 810 t·year–1. We consider the biological and economic consequences of alternative management approaches to the New Zealand orange roughy fishery. We suggest that given the uncertainty in stock abundance and productivity and market and processing capacity limits, the management of New Zealand orange roughy stocks has been close to economically optimal and has produced near maximum sustainable yield from the resource.

Multifrequency 12, 38, and 120 kHz acoustics were used to identify the dominant fish groups around a deepwater (>600 m) seamount (a known spawning site for orange roughy, Hoplostethus atlanticus) by amplitude mixing of the frequencies. This method showed three distinct acoustic groupings that corresponded to three groups of fishes based on size and swimbladder type: myctophids of total length less than 10 cm, morids and macrourids with lengths >30 cm, and orange roughy with a mean standard length of 36 cm. These three groups were the dominant groups caught in the demersal and pelagic trawls in the study area. A simple model of swimbladder resonance at depth of large and small gas-filled bladder fish groups is in agreement with our experimental observations. Traditionally, demersal and pelagic trawling is used to identify fish species in acoustic records. However, orange roughy are rarely caught in mid-water owing to net avoidance. Using three frequencies, these groups could be distinguished directly over their entire vertical extent from the acoustic records. This reduces a major source of positive bias uncertainty (factor range of 2.0–6.4) in the orange roughy biomass estimates.

Ages of orange roughy (Hoplostethus atlanticus) determined by two methods (counting annuli on the surface of whole and in longitudinally sectioned otoliths) were similar up to maturity. Beyond maturity, age estimates from sectioned otoliths exceeded those from whole otoliths. Maximum recorded age was 125 years for an individual 41 cm standard length (SL), and age at maturity was estimated to be 25 years (30–32 cm SL). These are consistent with ages estimated previously by radiometric methods. Results demonstrated a two-stage linear relationship between otolith weight and age that confirmed the two-stage otolith mass growth model previously used in radiometric ageing. However, in the radiometric analyses the reduction in otolith growth was arbitrarily estimated at 45% of the immature rate whereas annuli data demonstrated a reduction after maturity to 62% of the immature rate. The new estimates of otolith mass growth rate were incorporated into the radiometric data and ages recalculated, which reduced age estimates for 38–40 cm SL fish from 77–149 to 59–101 years. The radiometric data were also recalculated using only the percentage reduction in otolith growth after maturity, giving the radiometric age of 125 ± 9 years for the oldest fish.

Previous workers have used disequilibrium in the 210Pb: 226Ra ratio in fish otoliths to estimate the age of four teleost species: Hoplostethus atlanticus, Macruronus novaezelandiae, Sebastes diploproa, and Sebastes mentella. We argue that uncontrollable errors due to degassing of intermediate 222Rn from otoliths, unknown sources and sinks of 210Pb and 226Ra, possible changes in the ratio with which these isotopes are accreted to the otolith over the life of the fish, contamination by 210Po, difficulties in obtaining otolith cores, and the dependence of the estimated radiometric ages on the mass growth-in-time model usually adopted for otoliths can invalidate the application of disequilibrium techniques to fish age estimation.

Eleven polymorphic loci were screened in six southern Australian populations of the deepwater teleost Hoplostethus atlanticus, the orange roughy. Associations between heterozygosity and morphological variability were examined. No significant correlations between heterozygosity at individual loci and length were observed. Homozygotes at ADA* had significantly greater length variance than heterozygotes in one population whereas in another population, homozygotes for PGM-2* had significantly reduced length variance compared with heterozygotes. No other single locus/length variance comparisons were significant. One of the six populations showed a significant positive regression of individual length on multilocus heterozygosity; regressions in the other five populations were nonsignificant and variable in slope direction. No significant correlations were detected between multilocus heterozygosity category and mean length or coefficient of variance of length. Three of the populations were also examined for variation in four meristic characters and one measure of bilateral asymmetry: no consistent associations were detected between individual or multilocus heterozygosity and mean or variance of these characters. The data provide little evidence for significant relationships between individual or multilocus heterozygosity and morphological variability in orange roughy.

Risk analysis can enhance the value of scientific advice to fishery managers by expressing the uncertainty inherent in stock assessments in terms of biological risk. I present a case study involving an overexploited population of orange roughy (Hoplostethus atlanticus) on the Chatham Rise, New Zealand. This analysis quantifies the risk to the fishery and shows how this decreases as the rate of reduction in total allowble catch increases. The technique helps fishery managers balance biological risk against economic risk. Ways of generalizing the technique are discussed.